Electrostatic chucks are used for holding a workpiece in various applications ranging from holding a sheet of paper in a computer graphics blotter to holding a semiconductor wafer within a semiconductor fabrication process system. Although electrostatic chucks vary in design, they all are based on the principle of applying a voltage to one or more electrodes in the chuck so as to induce opposite polarity charges in the workpiece and electrode(s), respectively. The electrostatic attractive force between the opposite charges presses the workpiece against the chuck, thereby retaining the workpiece.
A problem with electrostatic chucks is the difficulty of removing the electric charge from the workpiece and the chuck when it is desired to release the workpiece from the chuck. One conventional solution is to connect both the electrode and the workpiece to ground to drain the charge. However, the charge trapped into the dielectric material cannot be drained freely. Another conventional solution, which purportedly removes the charge more quickly, is to reverse the polarity of D.C. voltage applied to the electrodes.
A shortcoming that has been observed with these conventional approaches to removing the electric charge is that they fail to completely remove the charge, so that some electrostatic force remains between the workpiece and the chuck. This residual electrostatic force necessitates the use of a large mechanical force to separate the workpiece from the chuck. When the workpiece is a semiconductor wafer, the force required for removal sometimes cracks or otherwise damages the wafer. Even when the wafer is not damaged, the difficulty of mechanically overcoming the residual electrostatic force sometimes causes the wafer to pop off the chuck unpredictably into a position from which it is difficult to retrieve by a conventional wafer transport robot. Wafer de-chucking is critical since it can impact particle generation and tool utilization should a wafer be broken or misplaced to the point where it requires that the chamber be opened to retrieve the wafer. These problems may be addressed by applying a de-chucking voltage to the chucking electrode to reduce or remove any residual electrostatic force holding the wafer when de-chucking the wafer. Determining the optimum de-chucking voltage is difficult.
The optimum de-chucking voltage provides for wafer lift-off without popping or significant robot corrections. Typically, an optimum de-chuck voltage is highly dependent upon the wafer characteristics and the plasma process conditions and the temperature of the electrostatic chuck.
The approach of the prior art was to apply a special de-chucking voltage when lifting off the wafer in order to counteract the residual chucking force and thereby avoid wafer breakage.
The foregoing methods are limited because the application and determination of an optimum de-chucking voltage varies among different plasma process conditions and different wafer designs and different electrostatic chuck designs. What is generally desired now is a de-chucking method that minimizes the residual chucking force upon wafer lift-off, regardless of variations in plasma process conditions, wafer characteristics and electrostatic chuck properties.